Radio transmitters convey information by encoding or “modulating” the information onto a radio “carrier” wave. Modulation of the carrier wave can take on many forms. Some types of modulation vary only the phase or frequency of the carrier wave. Other types of modulation vary the amplitude of the carrier wave resulting in a “non-constant envelope” modulation. The modulation information can be analog or digital and can be described as a “baseband” signal typically at a frequency lower than that the frequency of the carrier wave. Once modulated by one or more baseband signals, the modulated carrier wave is typically amplified by a power amplifier (“PA”) and transmitted, such as into free space, by a radio antenna.
A number of problems in conventional radio transmitter modulation techniques have been observed. For example, in a conventional transmit architecture of the prior art, baseband data are up-converted to a RF frequency by using a mixer and an oscillator. To achieve high data rate transmission, a high bandwidth signal needs to pass through the entire signal path which includes the up-conversion mixer and the PA. Also, non-constant envelope higher order modulations need to be used to achieve this high data rate. The up-conversion mixer and the PA need to be wide-band and at a same time very linear because of the requirements of non-constant envelope modulations. However, using a linear PA significantly reduces the PA power efficiency, a very important factor in any transmitter. Also, in conventional radio transmitter designs, there is a strong trade-off between high order non-constant envelope modulations and the transmitter power efficiency.
Another problem with conventional radio transmitter operation is that any receiving station that can receive enough power from the radio transmitter can generally demodulate the received carrier wave and have access to the modulation information. Some level of secure communications can be achieved by various encryption schemes. However, even with encryption of the information, there is always a risk that an unintended recipient might successfully decode the encrypted information.
There is a need for a transmitter architecture that can more efficiently modulate and transmit modulated radio carrier waves. There is also a need for a radio transmitter that can transmit modulated signals that can only be demodulated by an intended recipient radio receiver to which the transmitted signal has been directed.